1. Field of the Invention
The present invention relates to a gradient coil system, in particular for a magnetic resonance tomography apparatus, having a gradient coil layer and a layer structure that includes a cooling layer.
2. Description of the Prior Art
In the operation of a magnetic resonance tomography apparatus, for image production it is necessary to superimpose at least one gradient magnetic field on the magnetic basic field. This is done using a gradient coil system. Conventionally, the gradient coil system has a number of gradient coil layers having one or more gradient coils in order to produce three gradient magnetic fields perpendicular to one another. In the actively shielded gradient coil systems used today, in addition to the gradient coil layers that produce the useful field additional gradient coil layers are provided that externally shield the useful field. These additional gradient coil, layers are situated around the gradient coil layers that produce the useful field, at a distance from these layers.
Typically, a pulsed current on the order of magnitude of several hundred amperes flows through each gradient coil of the gradient coil layer. Due to the ohmic resistance of the gradient coils, a significant quantity of energy is converted into heat. This heat must be conveyed away in a suitable manner. In a magnetic resonance tomography apparatus for whole-body examinations, a good cooling of the gradient coils is particularly necessary because the patient is directly exposed to the heat produced by the gradient coil system.
Therefore, in a gradient coil system cooling layers are situated for conveying away the heat produced by the gradient coils of the gradient coil layers. The construction of such a cooling layer is described, for example, in DE 197 21 985 A1, and in WO 03/093853 A1.
For this purpose, one or more flexible cooling lines are situated on a gradient coil layer, so that the cooling lines essentially form a cooling layer between two layers of the gradient coil system.
The construction of such a gradient coil system is described in DE 100 32 836 C1. Additional details concerning the production of such a gradient coil system can be learned from DE 197 22 211 A1.
Here, the intermediate spaces between the individual gradient coil layers and the cooling layers are filled with a plastic matrix after the assembly of all the layers of the gradient coil system. In this way, the individual gradient coil layers and cooling layers are connected to form a unit. The plastic matrix, produced on the basis for example of an epoxy resin, has filling materials that have a high heat conductivity in order to achieve a good heat transfer from the gradient coil layer to the cooling lines of the cooling layer. In addition, the plastic matrix has elasticity, so that Lorentz forces that arise due to the very high currents between the individual gradient coils can be absorbed. In this way, at the same time the mechanical vibrations of the gradient coils during operation are damped, so that the production of noise by the gradient coil system during operation is relatively low.
Laying the cooling lines in a serpentine-like structure is very labor-intensive. In addition, the layers of the layer structure of the gradient coil system are usually concentrically situated hollow cylinders. The cooling lines thus must be situated on a curved surface. So that, before the casting with a plastic matrix, the cooling lines will remain in their serpentine-like structure, the individual cooling lines are sewed to the layer situated thereunder at various points. The sewing takes place using at least a plastic carrier onto which the gradient coils of the gradient coil layer situated thereunder are applied. This means an additional significant labor outlay. During the sewing, there is in addition the risk that a cooling line will be penetrated by a needle, and will thus develop a leak. This leak will normally not be recognized until a tightness test takes place after the manufacture of the gradient coil system has been completed. A single hole in a cooling line can make the entire gradient coil system unusable.